Various abbreviations that appear in the specification and/or in the drawing figures are defined as follows:
3GPP3rd Generation Partnership ProjectCPcyclic prefixDACdigital to analog converterDFTdiscrete Fourier transformFFTfast Fourier transformEUTRANevolved universal terrestrial radio access network(also LTE or 3.9G)IEEEinstitute of electrical and electronics engineersIFFTinverse Fast Fourier transformLTElong term evolution (3.9G)MSmobile stationOFDMorthogonal frequency division multiplexingOFDMAorthogonal frequency division multiple accessRFradio frequencyWiMAXworldwide interoperability for microwave accessUMTSuniversal mobile telecommunications systemUTRANUMTS terrestrial radio access network
OFDM is a multi-carrier data transmission technique that is advantageously used in radio-frequency based transmitter-receiver systems, such as for example 3GPP EUTRAN/LTE/3.9G and to IEEE 802.16d/e/WiMAX, IEEE 802.11a/WiFi, fixed wireless access (FWA), HiperLAN2, digital audio and digital video broadcast (DAB and DVB), and others. OFDM systems typically divide available radio spectrum into many carriers. Each of the many carriers has a narrow bandwidth and is modulated with a low rate data stream. The carriers are closely spaced and orthogonal separation of the carriers controls inter-carrier interference (ICI).
When generating an OFDM signal, each carrier is assigned a data stream and the data streams are converted to symbols based on a modulation scheme such as for example Quadrature Amplitude Modulation (QAM and its variants 16QAM, 64QAM etc), Quadrature Phase Shift Key (QPSK), and the like. Once the phase and amplitude are determined, they must be converted to time domain signals for transmission. Typically, OFDM systems use an IFFT to perform this conversion. The IFFT is an efficient way of mapping the data onto the orthogonal carriers. The time domain signal is then up converted to RF of the appropriate carrier and transmitted.
Delay in processing that signal in the transmitter is a concern for several reasons. First, for the case where the user equipment UE/mobile station MS is transmitting the signal, the symbols must be received at the receiver/BS/NodeB within a certain window, which the BS assures by sending a timing advance to the various MSs in its cell. Second and relatedly, multi-path delay interference in the transmitted symbols causes inter-symbol interference (ISI) between the reflected radio signal and the direct radio signal. A cyclic prefix CP, of which the time it occupies is referred to as a guard interval GI, is inserted to separate the symbols and avoid ISI, but inserting the CP must be done so that the transmitted signal is sent with the proper timing advance. To achieve a reasonable throughput, the OFDM symbol duration may be at least five times the GI, and to avoid ICI the OFDM symbol is cyclically extended in the GI.
CP insertion may be achieved as follows. An IFFT has an associated “length” corresponding to a number of coefficients for the transform. The CP is generated by placing the last few IFFT output coefficients at the beginning of the symbol to form the CP. The size of the CP varies in different applications, and a common CP length is ¼ the length of the IFFT (e.g., for an IFFT with a length of 64, the output corresponding to the last 16 coefficients may be transmitted first as the cyclic prefix, and then the output corresponding to the 64 coefficients of the entire IFFT is output in regular order).
For the addition of this CP in known systems, the IFFT output requires relatively large buffers. If the output of the IFFT is in bit-reversed time order, then two buffers of size N (N is length of IFFT) are required. If the output of the IFFT is in time order, then a single buffer of the length of the IFFT (N) is required. Buffers add expense to the system.
So at the transmitter side of an OFDM system, a CP is introduced into the transmitter signal after the IFFT. As noted above, typically this CP consists of some few last samples of an OFDM symbol which follows the cycle prefix as is shown in FIG. 1, where NIFFT indicates the number of samples of the symbol and NCP indicates the number of samples in the portion corresponding to the CP. Because of that CP insertion, an additional memory is needed to store the first large part of the OFDM symbol until the beginning of the CP sequence.
Especially for long OFDM symbols as used in LTE systems (2048 FFT or 2048 IFFT) the memory consumption and delay is remarkable and should be reduced. Some approaches have used a combination of interleaving and FFT to reduce the latency and complexity of the OFDM based system, but while this strategy may be successfully used at the receiver, they are not seen to reduce latency at the OFDM transmitter. At the transmitter side, CP insertion significantly degrades this latency gain. Since the last part of the IFFT symbol is used for the CP, all or almost of the overall symbol (NIFFT samples) has to be stored after the IFFT process block. This buffering adds an unwanted and unnecessary delay to the output of the IFFT output.
Since the IFFT output has an unusable order, the input symbol stream of the IFFT has to be modified to get the wanted IFFT output order. Mathematically, this problem can be solved by a frequency dependent phase shift of the sub-carrier symbols as shown in equation 1 below:
                                          ⅇ                                          -                j                            ⁢                                                2                  ⁢                  π                                                  N                  IFFT                                            ⁢                              kn                τ                                              ⁢                      X            ⁡                          (              k              )                                      =                                            ρ              ⁡                              (                                  kn                  τ                                )                                                    ︸              Phaserotation                                ⁢                                          ⁢                                    X              ⁡                              (                k                )                                      ⁢                          ⟶              IFFT                        ⁢                          x              ⁡                              (                                  n                  -                                      n                    τ                                                  )                                                                        [        1        ]            
In this equation, n denotes the time domain index and k denotes the frequency domain index. The IFFT output can be shifted by nτ time samples. For our purpose, nτ is equal to the number of IFFT samples (length NIFFT) minus the number of cyclic prefix samples Ncp(nτ=NIFFT−Ncp).
There has been some prior art research into the above problem of CP insertion and latency at the transmitter. For example, US Pat. Publ. No. US 2004/0120413 A1 entitled Multi-carrier transmission systems and methods using subcarrier relocation and guard interval insertion describes a multi-carrier transmission system which includes an encoder for converting a data sequence into encoded symbols corresponding to respective sub-carriers; a first shifter for rearranging the encoded symbols to define a guard interval length; an inverse fast Fourier transform (IFFT) unit for inverse fast Fourier transforming the rearranged encoded symbols; a second shifter for processing the transformed symbols to effect a frequency shift to compensate for a frequency shift effected by the IFFT unit; and a guard interval inserter for interleaving symbol replicas with the processed symbols according to the guard interval length.
Another reference relevant to the problem at hand is International Publication No. WO 2007/123340 A2, entitled Method and apparatus for inserting guard interval in a mobile communication system. This reference describes rotating a phase of each symbol for a stream, converting the phase rotated symbol stream into a time domain symbol stream, and copying a rear part of the time domain stream to a front part of the time domain stream and/or vice versa.
Another somewhat similar approach is seen as US Pat. Publ. No. US 2005/0047325 A1, entitled: Combined inverse fast Fourier transform and guard interval processing for efficient implementation of OFDM based systems. This document details an IFFT circuit that receives input data of length N coefficients and generates output data of length N coefficients that are circularly shifted by m coefficients. A CP insertion circuit inserts a CP of length m and includes a first switch connected to the IFFT circuit, a buffer of length m having an input connected to the first switch and an output, and a second switch, coupled to the first switch and to the buffer. The first and second switches selectively couple the output of the buffer and the IFFT circuit to an output of the second switch.